The present invention generally relates to cooling systems for electronic components. More particularly, this invention relates to a sealed cooling device with enhanced thermal management capabilities.
Cooling of electronic devices has become increasingly challenging as electronics have evolved. As manufacturing processes are constantly refined, the migration to smaller design processes and the incumbent reduction in operating voltage has not kept pace with the increased complexity of faster integrated circuits (ICs). Increasing number of transistors in combination with increasing operating frequencies has resulted in higher numbers of switching events over time per device. As a result, within the same market space and price range, ICs are becoming more and more sophisticated and power-hungry with every generation.
Compared to earlier generations, the implementation of smaller design processes has allowed the integration of more electronic building blocks such as transistors and capacitors on the same footprint. Consequently, area power densities have increased, resulting in smaller dies dissipating higher thermal load. As a result, formerly sufficient, passive heat spreaders and coolers often do not provide adequate cooling. While sophisticated fin designs and powerful fans increase the active surface area useable for offloading thermal energy to the environment, even extremely well designed coolers are hitting inherent limitations. In particular, significant limitations stem from the bottleneck of limited heat conductivity of the materials used, and specifically the fact that passive heat transfer throughout a solid structure is limited by the coefficient of thermal conductivity (CTE) of the material and the cross sectional area of the structure.
In a two-dimensional heat spreader of uniform thickness, the amount of thermal energy decreases as a square function of the distance from the source, where the thermal conductance coefficient of the material and the cross sectional area define the slope of the decrease. Therefore, even the most highly conductive material will not be able to maintain an even temperature distribution across the entire surface of the cooling device. Any gradient, on the other hand, will cause a decrease in cooling efficiency since the temperature difference (ΔT) between the cooler's surface and the environment is the primary limiting factor for thermal dissipation to the surrounding.
In view of the above, it is desired that coolers transfer heat from a heat source as quickly and efficiently as possible to optimize cooling efficiency for the heat source. In combustion engines, liquid cooling has become the method of choice, using the fact that a liquid (e.g., water) is taking up thermal energy and subsequently being pumped to a remote radiator where it releases the absorbed heat. In electronic devices, liquid cooling is still only marginally accepted for reasons that include the inherent risk of spills, cost overhead, and complexity of the installation, which involves routing of tubing and installation of radiators. Alternatively, some self-contained liquid cooling devices have been proposed and marketed.
Four primary factors defining the efficacy of a liquid cooling device are the uptake of heat by the cooling fluid at the heat source, the transport rate of the fluid away from the heat source, the offloading of heat to the solid components of the cooler, and finally the dissipation rate of heat into the environment. The exchange of heat between the heat source and the cooling fluid mainly depends on the surface area of the heat source that is exposed to the fluid in a direction normal to the plane of the heat source, for example, a semiconductor die. The exchange of heat between the fluid and the cooling device largely depends on the routing of the flow of the coolant within the device. If the channels are too wide, laminar flow can cause a decrease in efficacy of heat exchange between the fluid and the device. Therefore, it is desirable to have a capillary system to achieve an optimal surface to volume ratio. Such capillary systems have been referred to as microchannel systems.
One often overlooked problem with a nondescript microchannel system is that the hydraulics are poorly defined. If a pump simply pumps the fluid through the interstitial space without further routing in the form of macrochannels, then the centrifugal movement of the fluid can easily interfere with the centripetal flow that recycles the fluid back to the pump. A number of workaround possibilities have been proposed, among which is the separation of the centrifugal and the centripetal fluid movements into two individual planes that are separated by a septum. In other words, centrifugal flow of the fluid may occur within a lower plane whereas the centripetal “suction” of the fluid back to the pump may occur in an upper layer. This separates the centrifugal from the centripetal channel and as a consequence the pump does not have to work against itself.
A natural occurrence of such a “counter-flow” system is known as rete mirabile or wonder mesh in biomedical sciences. A relevant example in this context is the micro-vascularization in the feet of aquatic birds where, within less than one centimeter, the blood temperature drops in the arterial path from about 38° C. to the outside temperature, and then warms back up to body temperature in the venous path. The temperature change or rather exchange between the two paths occurs in an entirely passive manner, that is, without the addition of any extra energy. In other words, having liquids flow in opposite direction through adjacent capillary networks or microchannels can create the effect of a very efficient thermal isolation between two points. However, in the case of a cooling apparatus, such an effect is highly undesirable since heat would be trapped at its source despite all fluid movement. Consequently, it appears highly disadvantageous in a heat exchange apparatus to have flow and counter flow in immediate proximity, especially if the septum between outgoing and incoming channels is thermally conductive.
An online publication by C. Hammerschmidt, “IBM Technology Keeps Future Chips Cool,” EE Times Online, http://www.eetimes.com/news/semi/showArticle.jhtml;jsessionid=N3LUY22LJ4EB MQSNDLSCKHA?articleID=193402569 (visited 16 Nov. 2006), describes an approach by IBM using direct jet impingement. This technology uses water jets sprayed with several tens of thousands of micro-nozzles onto an integrated circuit as the primary cooling technique in combination with a tree-like branched return architecture also referred to as hierarchical channel system.
The use of microchannels for coolant fluids has been known for some time, as evidenced by U.S. Pat. No. 4,450,472 to Tuckerman et al. The preferred embodiment featured in this patent integrates microchannels into the die of a microchip to be cooled and coolant chambers. U.S. Pat. No. 5,801,442 also describes a similar approach. Still other approaches have focused on the combined use of coolant phase change (condensation) and microchannels, an example of which is U.S. Pat. No. 6,812,563. U.S. Pat. No. 6,934,154 describes a similar two-phase approach including an enhanced interface between an IC die and a heatspreader based on a flip-chip design and the use of a thermal interface material. U.S. Pat. Nos. 6,991,024, 6,942,018, and 6,785,134 describe electroosmotic pump mechanisms and vertical channels for increased heat transfer efficiencies. Variations of microchannel designs include vertical stacking of different orientational channel blocks as described in U.S. Pat. No. 6,675,875, flexible microchannel designs using patterned polyimide sheets as described in U.S. Pat. No. 6,904,966, and integrated heating/cooling pads for thermal regulation as described in U.S. Pat. No. 6,692,700.
Additional efforts have been directed to the manufacturing of microchannels. U.S. Pat. Nos. 7,000,684, 6,793,831, 6,672,502, and 6,989,134 are representative examples, and disclose forming microchannels by sawing, stamping, crosscutting, laser drilling, soft lithography, injection molding, electrodeposition, microetching, photoablation chemical micromachining, electrochemical micromachining, through-mask electrochemical micromachining, plasma etching, water jet, abrasive water jet, electrodischarge machining (EDM), pressing, folding, twisting, stretching, shrinking, deforming, and combinations thereof. However, these methods do not describe in any detail the geometry of the fluid circulatory system.
The use of a mesh to increase surface contact area with a cooling fluid has also been proposed. For example, meshes have been employed as condensers in evaporative cooling systems. Meshes have also been employed in the context of increasing contact with a cooling fluid used to cool semiconductor devices, for example, as described in U.S. Pat. No. 5,719,444 to Tilton et al. Commonly-assigned U.S. Pat. No. 7,219,715 to Popovich, the contents of which are incorporated herein by reference, describes an alternative approach using a mesh or woven screen that is between and bonded to two foils that define a flow cavity. With this approach, the interstices between the warp and weft strands of the mesh, as well as the gaps between the strands and the bordering foils, allow the passage of a cooling fluid, providing direct contact with the fluid for heat absorption and transfer heat through the bonding contacts with the foils. Both Tilton et al. and Popovich describe the semiconductor device being immersed in the cooling fluid.